This invention relates to an exposure apparatus for forming a desired pattern on a substrate such as a semiconductor substrate, for example, and also to a device manufacturing method using the same.
In the manufacture of devices with fine patterns, such as semiconductor devices (e.g., semiconductor integrated circuits), micromachines or thin film magnetic heads, for example, generally, light (visible light or ultraviolet light) or X-rays are projected through a mask to a substrate (workpiece to be exposed) by which a desired pattern is transferred onto the substrate. In the case of semiconductor integrated circuits, for example, a mask corresponding to a desired pattern is prepared and it is placed relative to a semiconductor substrate having a resist coating formed thereon. Then, light or X-rays are projected to the substrate through the mask, by which the resist material is selectively exposed to light, such that a circuit pattern is selectively exposed to light, such that a circuit pattern is transferred to the resist. Subsequently, through an etching process and a film formation process, for example, a desired circuit is produced on the semiconductor substrate. Taking the case of semiconductor integrated circuit manufacture as an example, production of a device having a very fine pattern such as described above, will be explained below.
The density and speed of such semiconductor integrated circuits are increasing more and more, and the linewidth of a pattern for the integrated circuit is narrowed more and more. In order to meet this, further improvements in the performance of semiconductor manufacturing methods are required. As regards printing apparatuses (exposure apparatuses), steppers which use an exposure light source of shorter wavelengths such as KrF excimer laser (248 nm), an ArF excimer laser (193 nm), or an X-ray region (0.2-15 nm), have been developed.
As regards a resist material to be used for the transfer of a desired resist to a substrate, on the other hand, a chemical amplification (intensifying) type resist using an acid catalyst has been developed.
With the narrowing of a linewidth of a desired pattern, dust-proofing measures become very difficult to accomplish. Not only does the limit to the size or number of dust particles become strict, but also the sensitivity of processes to chemical matters becomes higher. In a clean room in which semiconductor integrated circuits are produced, chemical contamination becomes a critical problem. This is attributable to decomposition products of a resist, scattered matters produced during a process such as development or washing, for example, or volatile matters generated from an adhesive agent or wall materials, for example.
When, in such a chemical environment, the exposure process using short wavelength light such as deep ultraviolet light or X-rays is performed for a long period, it causes contamination of the mask surface, that is, deposition of matters on the mask surface, which in turn causes a change in the light transmissivity, reflectivity or scatter characteristic of the mask. Particularly, when a chemical amplification type resist is used, an oxygen generating agent or acid and decomposition products are evaporated during the exposure process or after it, and contamination of the mask is accelerated.
FIG. 11 shows a reaction example of a chemical amplification type resist. A t-Boc (tertiary-butoxy carbonyl) group contained in the resist as an anti-dissolution agent is decomposed, whereby volatile butene is produced.
In an X-ray projection exposure process, a mask and a workpiece are exposed while a small gap of about several tens of microns is held therebetween. Therefore, contamination of the mask is a very serious problem. Depositions may have various shapes and compositions. While a certain tendency may be found, a definite casual relationship is not detected. It may be considered that the deposition is not based on a simple photochemical reaction but it results from complex functions of decomposition, recombination, multiple reaction, sediment, or crystallization, for example. If deposition is produced, it may be removed by washing. However, particularly in a case of an X-ray mask, washing is very difficult to do because the shape of an absorptive material has a high aspect. Practically, therefore, all of the dust particles cannot be removed by washing. Since, on the other hand, the supporting film comprises a very thin film, the strength is very low. Therefore, the washing operation could not be done frequently.
If the exposure process is repeated while mask surface contamination by dust or any other deposition is left there, a transferred pattern is influenced largely due to a reduction alignment light transmissivity and degradation of alignment precision, for example.
It is an object of the present invention to provide an exposure apparatus by which deposition or sediment of contaminants upon a mask surface can be prevented, to reduce or remove the necessity of mask washing operations and to prolong the lifetime of the mask.
It is another object of the present invention to provide an exposure apparatus by which degradation of exposure precision due to contamination of a mark surface can be prevented.
It is a further object of the present invention to provide a mask structure usable in such an exposure apparatus, an exposure method, a semiconductor device or a semiconductor device manufacturing method.
In accordance with investigations made by the inventors in regard to prevention of deposition or sediment of contaminants upon a mask surface, it has been found that contamination can be avoided by the following structure:
In an exposure apparatus for transferring a desired pattern formed on a mask structure onto a workpiece through an exposure process, the mask structure has a photocatalyst provided at least in a portion thereof, and an auxiliary chamber is defined therein for irradiating the mask structure with auxiliary light. The humidity in the auxiliary chamber may preferably be controlled.
A representative function of a photocatalyst is that various matters are schematically decomposed in response to irradiation of short wavelength light such as ultraviolet rays or X-rays. Further, it has an effect as that of a photo-semiconductor, and it is brought into a conductive state in response to irradiation with light, providing an anti-electrification function. Thus, it shows a function for preventing deposition of contaminants.
When the contamination of a mask reaches a certain level, the decomposition action may be accelerated in the auxiliary chamber. The auxiliary chamber may be controlled independently from an exposure ambience, and a light source and an environment best suited to the photocatalytic reaction may be prepared there. The auxiliary chamber may be provided with an exhaust port, for discharging decomposition products and for preventing redeposition of the same. This effectively increases the deposition decomposition action.
With the structure described above, the frequency of mask washing operation can be reduced, or the necessity of itself may be removed. Thus, the lifetime of the mask can be prolonged significantly.
In order that a photocatalyst functions as a catalyst, because of the band gap of the material thereof, it needs a large energy (short wavelength light) higher than a certain energy. However, if a metal is ion-injected, an absorptive band to light of longer wavelengths is produced, such that, with that absorptive band, a photocatalytic function is provided. Titanium oxide, which is a representative photocatalyst, absorbs light of wavelengths lower than 380 nm and produces a photocatalytic action. When Cr or V is ion-injected to the titanium oxide, while it depends on the amount of injection, light of longer wavelengths of about 450 nm can be used.
When a desired pattern is transferred to a workpiece in accordance with the present invention, the influence which otherwise might result from non-uniformness of exposure or insufficient control of exposure amount, due to contamination of a mask, or from degradation of alignment precision due to reduced alignment light transmissivity of the mask, can be avoided satisfactorily. Therefore, high precision printing can be done in mass production. Also, by printing a desired pattern on a workpiece in accordance with the present invention and then by processing the workpiece, high performance semiconductor devices can be produced, in mass production.